New paper: GenomegaMap for dN/dS in over 10,000 genomes

Published this week in Molecular Biology and Evolution, is a new paper joint with the CRyPTIC Consortium "GenomegaMap: within-species genome-wide d_N/d_S estimation from over 10,000 genomes".

The d_N/d_S ratio is a popular statistic in evolutionary genetics that quantifies the relative rates of protein-altering and non-protein-altering mutations. The rate is adjusted so that under neutral evolution - i.e. when the survival and reproductive advantage of all variants is the same - it equals 1. Typically, d_N/d_S is observed to be less than 1 meaning that new mutations tend to be disfavoured, implying they are harmful to survival or reproduction. Occasionally, d_N/d_S is observed to be greater than 1 meaning that new mutations are favoured, implying they provide some survival or reproductive advantage. The aim of estimating d_N/d_S is usually to identify mutations that provide an advantage.

Theoreticians are often critical of d_N/d_S because it is more of a descriptive statistic than a process-driven model of evolution. This overlooks the problem that currently available models make simplifying assumptions such as minimal interference between adjacent mutations within genes. These assumptions are not obviously appropriate in many species, including infectious micro-organisms, that exchange genetic material infrequently.

There are many methods for measuring d_N/d_S. This new paper overcomes two common problems:

• It is fast no matter how many genomes are analysed together.
• It is robust whether there is frequent genetic exchange (which causes phylogenetic methods to report spurious signals of advantageous mutation) or infrequent genetic exchange.

The paper includes detailed simulations that establish the validity of the approach, and it goes on to demonstrate how genomegaMap can detect advantageous mutations in 10,209 genomes of Mycobacterium tuberculosis, the bacterium that causes tuberculosis. The method reproduces known signals of advantageous mutations that make the bacteria resistant to antibiotics, and it discovers a new signal of advantageous mutations in a cold-shock protein called deaD or csdA.

Software that implements genomegaMap is available on Docker Hub and the source code and documentation are available on Git Hub.

With the steady rise of more and more genome sequences, the analysis of data becomes an increasing challenge even with modern computers, so it is hoped that this new method provides a useful way to exploit the opportunities in such large datasets to gain new insights into evolution.

Royal Society Summer Science Exhibition Stall July 2-8

Next week researchers from the Modernising Medical Microbiology consortium, collaborating groups and I will exhibit the Resistance is Futile stall at the Royal Society Summer Science Exhibition. The exhibition is a free event in central London open to all visitors. Our stall is an opportunity to tell visitors about our research, and how advances in genetics are influencing day-to-day life. On show at the Resistance is Futile stall:

Oxford Nanopore Technology Demos

DNA sequencing in the NHS is shortening the time to diagnose antibiotic resistance in serious infections

Evolution Dance Mat

Resistance mutants arise spontaneously through chance copying errors during DNA replication

Antibiotic Resistance Coconut Shy

Antibiotic use gives resistance mutants a strong advantage so they rapidly increase in frequency.

The exhibition runs from Monday 2 July - Sunday 8 July at Carlton House Terrace, London, SW1Y 5AG. For more information about our stall click here and for general visitor information about the exhibition click here. Please spread the word!

During the exhibition we will be tweeting from @ResistanceIF

Our stall is generously supported by Oxford Nanopore Technology, the Nuffield Department of Medicine, and through public engagement research funding awarded to our research groups by the Wellcome Trust, the Royal Society, the National Institute for Health Research, the Oxford Biomedical Research Centre, the Natural Environment Research Council, the Medical Research Council, the Newton Fund and the Bill & Melinda Gates Foundation.

New paper: Severe infections emerge from commensal bacteria by adaptive evolution

Published this month in eLife, our new paper on the evolution and adaptation of Staphylococcus aureus during infection.

This study shows that the emergence of life-threatening infections of the major pathogen Staphylococcus aureus from bacteria colonizing the nose is associated with repeatable adaptive evolution inside the human body.

First author Bernadette Young has summarized the paper's findings on the Modernising Medical Microbiology blog.

Nature Reviews Microbiology: Within-host evolution of bacterial pathogens

Our new review of what genomics has taught us about Within-host evolution of bacterial pathogens has been published in Nature Reviews Microbiology.

Cheltenham Science Festival

Earlier this week members of the group represented the Nuffield Department of Medicine at the Cheltenham Science Festival with our Modernising Medical Microbiology stall, featuring the Antibiotic Resistance Coconut Shy and the Genome Evolution Dance Mat.

Antibiotic Resistance Coconut Shy

Antibiotic Resistance Coconut Shy: The children (and adults) visiting the stall were given five bean bags (antibiotics) to throw at the coconuts (bacterial pathogens) to try to knock them off. The front row of coconuts, representing bacteria more susceptible to antibiotics, were easier to knock off than the back row, which represented more resistant bacteria. The aim was to show the children that an unwanted side effect of using antibiotics is to increase the frequency of resistant bacteria, because they were usually the ones left standing.

The game was more difficult than it looks, and just one visitor knocked off all five coconuts. We gave out NDM pens to the sixty visitors who managed to knock off three or more.

Microscope and Top Trumps

Digital Microscope: We brought along a light microscope to show the children what bacteria really look like, which helps emphasize how small they are since they are difficult to see even under the highest magnification. We prepared slides for several Gram positive and Gram negative species, and provided a key to help identify them. We also brought along a number of games that have been used in previous departmental outreach activities, including Pathogen Top Trumps and Fact or Fiction.

Genome Evolution Dance Mat

Genome Evolution Dance Mat: In this game, the children had to copy a bacterial DNA sequence by replicating a sequence of dance moves (up=A, left=C, right=G, down=T) without introducing new errors (mutations). Any mutations that were introduced were passed on to the next template sequence. In this way we aimed to show how mutations occur by errors in DNA replication, and that they are inherited. This generates unique DNA fingerprints for bacteria, which we can use to track the spread of outbreaks.

Outbreak Map

The game, which was kindly programmed by Gareth Jenkin-Jones, included a form of natural selection, so that if too many errors were introduced at once, the sequence was considered inviable and did not survive to be passed on. There was also a speed control, which was handy since some people appear to have spent a lot more of their youth playing dance mats than others.

Outbreak Map: We made an Outbreak Map to show the reach of our stall over the day, with visitors that scored highly on the coconut shy pushing in pins to show where they had travelled from. Had we been handing out germs instead of pens, we could have started outbreaks as far afield as Edinburgh, France and Spain, as well as a large cluster in Cheltenham and the surrounding counties.

Other research groups are representing the department throughout the week.

NDM Microbiology Stall at the Cheltenham Science Festival (L-R): Sarah Earle, Louise Pankhurst, Danny Wilson, Liz Batty, Dilrini De Silva, Jess Hedge, Catrin Moore. Amy Mason, Gareth Jenkin-Jones and Jane Charlesworth also helped with the preparations, and Jen Bardsley co-ordinated all the NDM Stalls.

New paper: Mobile elements drive recombination hotspots in the core genome of Staphylococcus aureus

This week published in Nature Communications we have a new open access paper looking at what drives variability in rates of recombination (horizontal gene transfer, HGT) in the core genome of Staphylococcus aureus. HGT in the core genome is important for eliminating harmful mutations and promoting the spread of beneficial mutations, such as those that make the bacteria resistant to antibiotics.

Compared to recent work focusing on individual, highly-related strains of S. aureus, we found much higher rates of core HGT across the species as a whole. We saw that the frequency of HGT varies along the genome. At broad scales, core HGT is higher near the origin of replication, a pattern reminiscent of the one described by Eduardo Rocha and colleagues in E. coli, who hypothesized that the over-abundance of DNA near the origin during rapid growth could promote HGT.

At fine scales, we found more frequent HGT in regions of the core genome close to mobile elements. The hottest regions occurred near mobile regions called ICE6013, SCC and genomic island α. The insertion and excision of mobile elements from the genome represents a type of HGT, so our finding that nearby core regions also experience more HGT suggests there is some sort of "spill over". This idea is supported by work in Ashley Robinson's group that found similarities between ICE6013 and a class of mobile elements in Streptococcus agalactiae called TnGBS2. TnGBS2 was discovered by Phillipe Glaser's lab who showed it sometimes transfers large tracts of adjacent core material during conjugation.

Whether conjugation alone can explain the high levels of core HGT we saw in S. aureus is unclear - our results suggest there is detectable HGT even in core regions far from mobile elements. Transformation is another possible mechanism of core HGT, but S. aureus is generally thought to be naturally incapable of transformation. However, intriguing work published by Tarek Msadek and colleagues in 2012 indicates there may be cryptic mechanisms of transformation in S. aureus after all. It remains to be seen whether the relative contributions of transformation, transduction and conjugation to the long-term evolution of S. aureus can be disentangled.

Within-host evolution of Staphylococcus aureus during asymptomatic carriage

Given its notoriety as one of the world's major causes of infection-related deaths, it may come as a surprise that one in three healthy adults carry the human pathogen Staphylococcus aureus in their noses without adverse effects. Indeed, most people carry the bacteria at some point in their lives. So carriage must be seen as the normal state of affairs in the human-S. aureus interaction, and by understanding this state better we can improve our understanding of why, in some people, the bacteria go on to cause life-threatening invasive disease.

This month sees publication of an investigation by my colleagues and me into the evolution of S. aureus during this normal healthy carriage state. The carriers in our study harboured populations of the bacteria that were very closely related but typically not identical, implying that the bacteria had evolved within the human body. The nose appears to be a microcosm of evolution for S. aureus, showing all the different types of genetic variation known at the species level within the noses of these individual carriers. For the most part, within-host evolution of the bacteria was very conservative, but certain proteins expressed on the surface of the bacteria and toxins secreted by the bacteria showed evidence of involvement in a host-pathogen arms race.

The paper, whose lead authors include Tanya Golubchik, Liz Batty, Derrick Crook and Rory Bowden, has received coverage on the EveryONE blog and F1000. I liked Gerald Pier's conclusion, made on the post-publication peer review website: "Given that about 30% of the world's seven billion-plus humans, and an unknown number of animals, are chronically colonized with S. aureus, the tremendous opportunity provided to this organism for generating genetic variation to counteract human efforts to prevent S. aureus infections may be one of the most formidable barriers to overcome in order to develop vaccines and highly effective interventions to lessen the impact of this organism on human and animal health."